Power lines form an important part of the power distribution system, carrying power from generation facilities all the way to the locations where it is used. The power distribution system may include many types of power lines, with high voltage lines used closer to the power generation facilities and medium and lower voltage lines closer to the locations where the power is used. These lines may run overhead. Alternatively power lines, particularly medium voltage lines, may run underground. Many commercial buildings, for example, receive power through underground power lines.
A power company may desire to obtain accurate voltage measurements of power lines in its power distribution system. Measurements may be used for multiple purposes, such as to detect or predict faults in the lines or to manage the power distribution system. For example, voltage measurements may be used to manage voltage levels and the amount of reactive power throughout the power distribution system (e.g., by using the measured voltage to determine how to operate switched capacitor banks and/or other components of a power distribution system). As another example, voltage measurements may be used to detect power theft.
Measuring voltage on underground wires poses special difficulties. Though underground wires are generally used for medium voltage, they might nonetheless carry tens of thousands of volts. For safety reasons, these lines are heavily insulated to prevent humans from accidentally contacting conductors at that voltage. Conventional approaches to measuring the voltage of underground power lines involve a voltage test port that enables access to the line through the insulation. Such a test port may be built into the cable connectors, elbows, or splices and allows a probe to be brought near enough the power line for a voltage to be capacitively coupled to the probe. The probe can then measure the capacitively coupled voltage, which indicates the voltage on the power line.